A fundamental problem for behavioral neuroscience is to understand physiological control of individual variation in reproductive behavior and aggression. Progress in explaining sexually dimorphic behavior illustrates that this type of problem is most tractable in systems with discontinuous behavior variation. This application proposes to test the generality of the organization-activation model of hormonal control, which has been successfully applied to hormonal control of sexually dimorphic behavior, to situations of similarly discontinuous within-sex behavior variation. The most common example of the latter occurs in species with alternative male reproductive tactics. The hypothesis proposed predicts that the role that hormones play will depend on whether alternative male phenotypes are fixed or plastic. For fixed phenotypes, hormones should play a greater role during development (organization) than during adulthood (activation). The model system chosen, the tree lizard, has a fixed male behavioral polymorphism. Prior to sexual maturity, males permanently develop into one of two phenotypes. One phenotype is highly aggressive and territorial, whereas the other is less aggressive and appears to be nonterritorial and nomadic in nature. This proposal continues our investigation of organizational actions of hormones in the differentiation of these two phenotypes, particularly advancing our investigation of a heretofore unsuspected role for progesterone during early male differentiation which was discovered in the previous grant and which also may be significant in more typical species. In addition, we propose to investigate the hormonal basis of plastic tactics recently discovered within the less aggressive phenotype to capitalize on the rare opportunity to test both sets of complementary predictions of the above hypothesis in the same species. Finally, we propose to expand our studies to a more typical, monomorphic species to test the generality of the findings in the polymorphic species. Together, the studies provide one of the first and most complete descriptions of the role of hormones in within-sex behavioral differentiation, test the generality of the organizational-activational model and provide basic information useful in understanding behavioral variation, both continuous and discontinuous, in all species, including humans.